Various analytical procedures and devices are commonly employed to determine the presence and/or concentration of analytes that may be present in a test sample. For instance, immunoassays utilize mechanisms of the immune systems, wherein antibodies are produced in response to the presence of antigens that are pathogenic or foreign to the organisms. These antibodies and antigens, i.e., immunoreactants, are capable of binding with one another, thereby causing a highly specific reaction mechanism that can be used to determine the presence or concentration of that particular antigen in a biological sample.
In many assay devices, labeled particles are used to signal the presence or absence of the analyte of interest, either visually or though the use of an instrument. For instance, commercial examples of fluorescent carboxylated microspheres are available from Molecular Probes, Inc. under the trade names “FluoSphere” (Red 580/605) and “TransfluoSphere” (543/620). Commercial examples of colored carboxylated latex beads are also available from Bang's Laboratory, Inc. Gold particles are also commonly utilized.
Despite some success, conventional particles still possess numerous problems when used in assay devices. For instance, conventional latex beads tend to aggregate with each other, thus requiring that they be refrigerated and kept under continuous agitation until use. This aggregation may lead to poor reliability and reproducibility in the assay device. Further, although gold particles have a relatively lower size distribution and do not tend to aggregate as much as latex beads, they are difficult and expensive to manufacture. Many of the commercially available gold particles are also poor in quality. Moreover, most gold particles have a red color that may not be changed, leading to less flexibility in the assay format and design.
As such, a need currently exists for improved particles for incorporation into an assay device.